The semiconductor industry continuously seeks methods to reduce the cost per unit wafer processed. Factors affecting wafer processing costs include failure rates (e.g., arising from wafer contamination by particles, improper wafer transfer or placement) equipment costs, and clean room costs, among others.
The wafer handler dictates a large percentage of wafer processing costs due to the relative cost and complexity of the wafer handler and the large clean room area wafer handlers need for operation (i.e., their axis of rotation). Therefore, a wafer handler must be designed to reduce particle generation and/or to prevent particles from becoming air born, as well as to provide accurate-repeatable wafer placement.
A particularly advantageous wafer handler configuration, known as a "frog-leg" robot is disclosed in U.S. Pat. Nos. 5,447,409 and 5,227,708 (the entirety of both references is hereby incorporated by reference herein). As described therein such frog leg robots operate in a single plane, providing highly accurate wafer placement, and allowing frog leg robots to be driven by low-particle magnetic couplings. A conventional frog-leg robot is described below with reference to FIGS. 1A-E and 2.
FIGS. 1A-E are schematic top views sequentially showing the operation of a conventional frog-leg robot 11. The frog-leg robot 11 comprises a central hub 13 about which a first arm 15 and a second arm 17 rotate. The first arm 15 comprises a first upper portion 19 rotatably coupled to the central hub 13, and a first lower portion 21 rotatably coupled to the first upper portion 19 (forming a first elbow 23) and a first hand 25 rotatably coupled to the first lower portion 21 via a first wrist 27. Similarly, the second arm 17 comprises a second upper portion 29 rotatably coupled to the central hub 13, a second lower portion 31 rotatably coupled to the second upper portion 29 (forming a second elbow 33) and a second hand 35 rotatably coupled to the second lower portion 31 via a wrist 37. A blade 39 is coupled to both the first hand 25 and the second hand 35 at a distance half way between the first and second wrists 27, 37.
As shown in FIGS. 1A-E a wafer 41 is positioned on the blade 39, and the central hub 13 is positioned in front of the processing chamber 43 which encloses a wafer placement location 45. The processing chamber 43 comprises a wafer exchange port 47 through which the frog-leg robot 11 may transport wafers to and from the wafer placement location 45.
In order for the frog-leg robot 11 to extend from a retracted position (FIG. 1A) past a center position (FIG. 1C) to a placement position (FIG. 1E) the frog-leg robot 11 must be configured to satisfy the following equation: EQU L.sub.upper .ltoreq.L.sub.lower +1/2D.sub.wrist {equation 1}
wherein:
L.sub.upper is the length of the upper portion of the first arm and the length of the upper portion of the second arm; PA1 L.sub.lower is the length of the lower portion of the first arm and the length of the lower portion of the second arm; and PA1 D.sub.wrist is the distance between the first wrist and the second wrist.
The need for the frog-leg robot 11 to satisfy the above equation is explained below with reference to the operation of the frog-leg robot 11.
In operation, the frog-leg robot 11 picks up the wafer 41 from a first location (not shown). Thereafter the frog-leg robot 11 assumes a retracted position and, while in the retracted position, rotates (e.g., 180.degree.) to position the wafer 41 in front of the wafer exchange port 47 as shown in FIG. 1A.
To assume the retracted position the first upper portion 19 rotates clockwise and the second upper portion 29 rotates counterclockwise about the central hub 13, drawing the first elbow 23 and second elbow 33 backward (i.e., away from the processing chamber 43). Because the first arm 15 and the second arm 17 are coupled in a closed loop (e.g., because the first arm 15 and the second arm 17 are coupled via the first hand 25 and the second hand 35 and are both coupled to the central hub 13), the first lower portion 21 and the second lower portion 31 rotate about the first elbow 23, the first wrist 27 and the second elbow 33 and the second wrist 37, respectively, as the first elbow 23 and the second elbow 33 draw backward. Thus, in the retracted position the frog-leg robot 11 assumes its minimum overall length (i.e., has a length equal to the length of the lower arms 21, 31, and the length of the blade 39). As the retracted frog-leg robot 11 rotates, for example from the first location to alignment with the wafer exchange port 47, it occupies a minimum axis of rotation indicated by the dashed circle 49.
After the frog-leg robot 11 is aligned with the wafer exchange port 47, the first upper portion 19 rotates counterclockwise while the second upper portion 29 rotates clockwise about the central hub 13, causing the frog-leg robot 11 to reach forward (i.e., toward the processing chamber 43) until the frog-leg robot 11 assumes the object placement position as shown in FIG. 1E. Because the first arm 15 and the second arm 17 are coupled in a closed loop, the first lower portion 21 and the second lower portion 31 rotate about the first elbow 23, the first wrist 27 and the second elbow 33 and the second wrist 37, respectively.
The frog-leg robot 11 is shown sequentially in FIGS. 1A-E moving from the retracted position to the placement position. FIG. 1B shows the frog-leg robot 11 as it moves from a retracted position toward a center position, the first elbow 23 and the second elbow 33 being backward of the central hub 13. FIG. 1C shows the frog-leg robot 11 in a center position, with the first elbow 23 and the second elbow 33 directly in line with the central hub 13. FIG. 1D shows the frog-leg robot 11 as it moves from the center position toward the placement position, the first elbow 23 and the second elbow 33 being forward of the central hub 13. Finally, FIG. BE shows the frog-leg robot 11 in the placement position with the first elbow 23 and the second elbow 33 in line with the first wrist 27 and the wrist 37, respectively.
Because the first arm 15 and the second arm 17 are coupled in a closed loop, and because the first hand 25 and the second hand 35 are maintained perpendicular to the blade 39 (due to constraint of suitable mechanisms within the wrist--e.g., gears or belts), the frog-leg robot 11 must satisfy equation 1 in order for the frog-leg robot 11 to be capable of assuming the positions shown in FIGS. 1B-D. Moreover, in order to reach the wafer placement location 45, both the hands 25, 35 and the wrists 27, 37 must extend through the wafer exchange port 47, as best shown in FIG. 2.
FIGS. 2A and 2B are a schematic side elevational view and a schematic top plan view, respectively, of the wrists 27, 37 and the blade 39 of the frog-leg robot 11, extending through the wafer exchange port 47 to the wafer placement location 45. The wrists 27, 37, due to their moving parts (not shown) and the case 51 which encloses the moving parts (and encloses particles generated thereby) has a thickness (indicated by the arrow "A") and a width (indicated by the arrow "C") which are substantially greater than the thickness of the blade 39 (indicated by the arrow "B") and the width of the wafer exchange port 47 (indicated by the arrow "D"). Accordingly the wafer exchange port 47 must be wider than the case 51. Such large wafer exchange ports are undesirable because they may allow more particles to enter the processing chamber. Further, the case 51 may not completely isolate particles generated by the wrists and thus the robot 11 may be responsible for emitting particles within the processing chamber. Additionally, the wrists' precisely machined moving parts may be adversely affected by the atmosphere within the processing chamber 43 (e.g., by the chamber's temperature, pressure, chemicals, etc.). While a longer blade may successfully isolate the wrist from the processing environment, the inclusion of a longer blade would cause a significant processing cost increase. The dashed circle 49 would increase, requiring a larger clean room area for robot operation.
Accordingly, although frog-leg robots are desirable for their accurate, repeatable and inexpensive design, as well as for their low particle generation, a need exists for an improved frog leg robot that will allow the wrist to remain outside the process chamber without increasing the minimum axis of rotation.